Moreover, ris has been combined with different techniques in the literature. For instance, the RIS-aided mmwave system was considered in [1]} and [2]}, where the authors discussed the snr maximization problem and the joint hybrid precoding design, respectively. In [3]} and [4]}, the application of ris in wideband ofdm systems was discussed to maximize the sum-rate. Moreover, in [5]} and [6]}, the authors considered the cooperation of bss and ris in a multi-cell system. How to exploit ris to enhance the performance of noma systems was discussed in [7]}. In addition, the secure transmission via ris has been investigated in [8]}, and ris was also considered to assist the communication using uavs [9]}.
